This invention relates to a method for evaluating drilling conditions while drilling a borehole. More particularly, this invention relates to a method for evaluating formations and bit condition while drilling. Further, this invention relates to a method for providing drilling alerts when inefficient drilling conditions are identified.
A drill string generally has a lower portion which is comprised of relatively heavy lengths of uniform diameter drill collar. A drill bit is attached to the downhole end of the drill collar, where a portion of the weight of the collar is applied causing the bit to gouge and crush into the earth as the drill string is rotated from the surface (e.g., a rotary table with slips). Alternatively, a downhole motor is employed to rotate the bit. The downhole motor is generally employed in directional drilling applications.
Measurement-while-drilling (MWD) systems are known for identifying and evaluating rock formations and monitoring the trajectory of the borehole in real time. An MWD tool is generally located in the lower portion of the drill string near the bit. The tool is either housed in a section of drill collar or formed so as to be compatible with the drill collar. It is desirable to provide information of the formation as close to the drill bit as is feasible. Several methods for evaluating the formation using the drill bit have been employed. These methods eliminate the time lag between the time the bit penetrates the formation and the time the MWD tool senses that area of the formation. The measurements available are rate of penetration (ROP) and bit revolutions per minute (RPM) which are determined at the surface and, downhole weight on bit (WOB) and downhole torque on the bit (TOR) which are derived from real time insitu measurements made by an MWD tool. WOB and TOR may be measured by the MWD tools described in U.S. Pat. Nos. 4,821,563 and 4,958,517, both of which are assigned to the assignee hereof.
Methods employing ROP, RPM, WOB and TOR measurements have been developed to determine certain formation characteristics at the drill bit. One such method is disclosed in U.S. Pat. No. 4,883,914 to Rasmus. The Rasmus patent employs the aforementioned measurements (i.e., ROP, RPM, WOB, and TOR), a gamma ray measurement and a resistivity measurement to detect an overpressure porosity condition. The gamma ray and resistivity measurements are included in order to account for the volume of shale and the apparent resistivity in the formation. It is known that an overpressure condition occurs when water is trapped in a porous formation (i.e., overburden). This overburden condition prevents the shale in the formation from further compaction, whereby the compressive stress is transmitted to the interstitial water. Therefore, this portion of the formation will have a supernormal pressure when compared to that of the surrounding formation. The method of U.S. Pat. No. 4,883,914 employs this overpressure porosity to determine desired drilling mud pressure, pore pressure (i.e., formation pressure) and formation strength.
U.S. Pat. No. 4,852,399 to Falconer discloses a method for distinguishing between argillaceous, porous and tight formations by computing formation strength from ROP, RPM, WOB and bit diameter (D). The formations are distinguished by setting upper and lower shale limits.
European Patent No. EP 0351902A1 to Curry et al discloses a method for determining formation porosity from WOB and TOR measurements which factor in the geometry of the drill bit.
U.S. Pat. No. 4,697,650 to Fontenot discloses a method of compiling a history of ROP, RPM, WOB and TOR measurements. U.S. Pat. No. 4,685,329 to Burgess discloses a method of compiling a history of TOR/WOB and ROP/RPM based ratios in order to identify trends such as bit wear, pore pressure variation and changes in lithology.
U.S. Pat. No. 4,627,276 to Burgess et al discloses a method for determining wear of milled tooth bits from a bit efficiency term which is derived from ROP, RPM, WOB and TOR measurements and bit geometry.